1. Field of the Invention
This invention relates in general to a protective relaying system and, more particularly, to a protective relaying system utilizing a digital computer for protecting an electric power system.
2. Description of the Prior Art
Recently a protective relaying system using a digital computer, such as a mini-computer or a microcomputer, has been developed which utilizes modern digital techniques. In such a protective relaying system using a digital computer, hereinafter referred to as a digital protective relaying system, an electrical quantity in an electric power system is converted into a digital quantity and a relay decision is made by processing the digital quantity in the digital computer. FIG. 1 is a block diagram illustrating one example of a conventional digital protective relaying system 100. In FIG. 1, a plurality of information quantities regarding voltages and currents of the electric power system are applied to input transformers (not shown) so as to be converted into signals Vi of suitable amplitude for use by filters 1. The signals Vi are applied to the filters 1 so that the fundamental wave components thereof are picked up. The output signals of the filters 1 are applied to sample and hold circuits 2 where the signals are held for a predetermined time and simultaneously sampled. The output signals of the sample and hold circuits 2, the so-called "sampled data", are applied to a multiplexer 3 to be selected consecutively and to be sent serially to an analog to digital converter 4. The applied serial signals are converted into digital signals in the analog to digital converter 4 and applied to a central processing unit 6 through a direct memory access control circuit 5. The central processing unit 6 executes a relaying calculation on the applied data expressing the information regarding voltages and currents of the electric power system, and produces a relay output signal Vo according to the results of the relaying calculation.
The digital protective relaying system 100 shown in FIG. 1 can execute relay decisions regarding many elements. For example, a distance protective relaying system for transmission lines having up to approximately one hundred elements can be embodied in the digital protective relaying system 100 shown in FIG. 1. Moreover, the digital protective relaying system 100 is advantageous in that standardization of hardware can be promoted because modification of the method of protection can be achieved by modifying the program of the digital computer. However, a malfunction in a protective relaying system for protecting an electric power system has an extremely large effect on the electric power system and thus can cause serious problems. To avoid this, a protective relay for fail safe use (hereinafter referred to as a fail safe relay) is provided in the protective relaying system. For example, in a distance protective relaying system, an overcurrent relay, an overvoltage relay, and an undervoltage relay are used as fail safe relays. Most of these fail safe relays are of a type such that a single electrical quantity is utilized in making a relay decision and that relay decision is executed by simple processing. Therefore, in the case of a protective relaying system where a single electrical quantity is utilized in making a relay decision, such as a fail safe relay, using a digital computer, the protective relaying system having a construction as shown in FIG. 1 has more processing capacity than necessary. This means that the system has more circuitry than it needs and thus is wasteful. Moreover, the digital protective relaying system with more circuitry than necessary is not only expensive but also has an unnecessarily high power consumption which generates much heat. The heat has an adverse effect on the digital computer which is mainly made of integrated circuits. Moreover, it is not good from a reliability standpoint to use many parts in the digital protective relaying system.
As described above, according to conventional techniques, it has been very difficult to construct a digital protective relaying system having suitable processing capacity where a single electrical quantity is utilized in making a relay decision, such as a fail safe relay, using a digital computer.